1. General
APPENDIX B
Protection of Catalyst Filled Reactors
Most pressure vessel codes and regulations require that a vessel be protected against equipment
overpressure by positive-action, code-approved protective devices. Overpressure is considered as a
pressure greater than the maximum allowable working pressure, which is generally the design pressure
that is stamped on the vessel nameplate. Pressure relief valves are provided to protect an individual
vessel or a system of vessels.
2. Flow Resistance
2.1 Catalyst-filled reactors usually are protected by a pressure relief valve on a gas-liquid separator
downstream of the reactor. This is a viable system, provided that no appreciable flow resistance arises
between the reactor inlet and the separator.
2.2 Flow resistance could build up due to:
a. Plugging of the catalyst bed
b. Fluid distribution or collection devices
c. Piping between reactor and separator
2.3 Experience has shown that whenever a buildup of flow resistance occurs it takes place over time
periods longer than the commonly accepted time period of 10–30 minutes, provided for operator response
to an abnormal condition. Therefore, high-pressure alarms shall be placed upstream of the reactors to
alert the operators to take appropriate action when the pressure at this point becomes hazardous.
3. Special Relief Devices
To provide structural reliability in the unlikely event of failure of operator response or complete failure of all
alarms or other types of protective devices, the reactor vessels shall be designed for a pressure not less
than 83 percent of the maximum source pressure.
a. When the reactors are designed for less than 83 percent of the maximum source pressure,
additional special pressure relief devices are required. Each case shall be considered individually.
b. In some instances, although reactors have been designed for pressures above 83 percent of the
source pressure, additional special pressure relief devices shall be required when they are subject to
exothermic reactions.
APPENDIX C
Dynamic Pressure Relief System Analysis
a. A dynamic pressure relief system analysis is conducted by building an engineering grade
dynamic model of a tower as well as associated controls, pumps, drums, reboilers, condensers,
pressure relief valves. Flare loads are then computed by imposing the same upsets on the model
which will cause flaring in the plant (for example loss of power causing loss of pumps and loss of
fin-fan motors).
b. The results obtained are more realistic than those obtained via classical steady state design
methods in two ways. First, the dynamic analysis results more accurately reflects the sequence and
duration of relief from pressure relief devices. For example, the dynamic analysis of multiple PSVs on
a tower show when each PSV lifts, how long it stays open and its associated flare flow. Historically,
these studies have found that PSVs do not lift simultaneously and the peak load is invariably
significantly less than the sum of all of the PSV flowrates.
c. In addition, a dynamic model eliminates “double counting”. When a PSV removes material from
several interconnected vessels, other PSVs may not lift due to reduced inventory.